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Use the Intelligent Digitizer

Use the Intelligent Digitizer

When extracting features, it is best to create separate layers for different feature types. For example, create one layer for roads, another layer for creeks, and so forth. The Intelligent Digitizer parameter defaults are set to provide results appropriate for road centerline extraction; however, you may want to adjust the parameters before extracting different feature types.

  1. To use intelligent mode, ensure the following are enabled in the Vector Parameters menu bar. (If you accessed Intelligent Digitizer using Vector > Intelligent Digitizer, these options are already set.)
    • Mode > Add New Vectors
    • Mode > Polygon or Polyline
    • Mode > Multi Part: Off
    • Mode > Intelligent Digitizer

  2. In the Window field, select which display group window to use for extracting features. The choices are Image, Scroll, or Zoom. To disable feature extraction in all windows, select Off.
  3. Left-click to specify the first seed point of the feature to extract, and continue to add seed points by left-clicking at intervals along the feature. ENVI automatically connects the seed points as you go. See Intelligent Digitizer Mouse Button Functions. Some general guidelines for extracting features are as follows:
    • For road centerline extraction, select seed points near the road centerline.
    • For features that contain sharp curves, select seed points at the sharp curve.
    • If the spectrum of the feature surface changes abruptly, select a seed point a few pixels beyond the surface change.
    • For polygon extraction, define the last seed point of the polygon on or near the first seed point. If the first and last seed points are too far apart when you accept the polygon, ENVI connects the nodes with a straight line, rather than following the shape of the feature.
    • If a seed point does not extract the feature as desired, middle-click to delete the seed point and select a new one. Clicking on a location closer to the previous seed point will likely give a better result.
    • To extract polyline features that will eventually intersect with features you have not yet extracted, extend the start or end node beyond where the features will intersect. You can correct overshooting polylines afterward using automatic post-processing.
    • In areas that do not provide sufficient contrast between the feature and its background, press and hold the Shift key to toggle to standard vector mode, then left-click to define the feature through the area of low contrast. When back to an area of higher contrast, release the Shift key to resume using intelligent mode.
  4. When there are enough seed points to extract the feature, right-click to set the polygon or polyline placement. A diamond-shaped handle appears near the polygon or polyline. If needed, you can left-click on the diamond and drag it to change vector placement.
  5. Right-click again and select one of the following:
    • Accept New Polyline/Polygon: Accepts the placement of the polyline or polygon.
    • Remove New Polyline/Polygon: Deletes the polyline or polygon.
    • Snap Start Node to the Nearest Polyline: Accepts the placement of the polyline and connects its start node with an existing nearby polyline.
    • Snap End Node to the Nearest Polyline: Accepts the placement of the polyline and connects its end node with an existing nearby polyline.
    • Snap Both Ends to the Nearest Polylines: Accepts the placement of the polyline and connects both its start and end nodes with existing nearby polylines.

      Tip: For snapping options, the polylines to snap must be within the Snap Tolerance (Pixels) parameter in the Intelligent Digitizer Parameters dialog.

  6. Perform automatic and/or manual post-processing operations after the feature extracting is complete.

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